There is conventionally known a valve timing controller for shifting a relative rotational phase between two rotary elements, which rotate respectively in response to the crankshaft and the camshaft, by a differential gear system composed mainly of a planetary gear. DE-4110195C2 discloses a valve timing controller which includes a sprocket rotating in response to a crankshaft, and a gear element rotating in response to the camshaft. The valve timing controller has a differential gear system composed mainly of a planetary gear between the sprocket and the camshaft. This valve timing controller converts a planetary motion of the planetary gear into a relative rotational motion of the camshaft to the sprocket. Two rotary elements include two internal gears, and the planetary gear has two external gears. These external and internal gears respectively engage with each other at one part of teeth thereof.
There is, therefore, a possibility that the planetary gear is inclined comparatively largely in the thrust direction by transmission of the torque by the planetary motion of the external gear part and the internal gear part. In particular, when the relative rotational movement is performed in high velocity, there is a possibility that a minor irregular friction may be caused by colliding of the gear part with the sprocket. As a result, because of the generation of the minor irregular friction, there may be a possibility that the normal relative rotational movement state is not maintained.
Furthermore, since the gear part and the sprocket collide with each other in a state both of them being in inclined state, as described above, a stress is locally increased at the meshing teeth each other, causing a factor of abrasion or damage.